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Reducing Scan Velocity Range in a Large-Depth-Of-Field Scanner

Publishing Venue

IBM

Related People

Dickson, LD: AUTHOR

Abstract

This article describes a means for introducing a scan angle reduction factor in a holographic scanner by directing the diffracted beam toward the axis of rotation instead of away from it. This reduction factor will allow the holographic scanner to achieve a much larger depth of field than would otherwise be possible because of the resultant reduction in the total scan velocity range, and the accompanying reduction in the required electronic bandwidth.

Country

United States

Language

English (United States)

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Reducing Scan Velocity Range in a Large-Depth-Of-Field Scanner

This article describes a means for introducing a scan angle reduction factor
in a holographic scanner by directing the diffracted beam toward the axis of
rotation instead of away from it. This reduction factor will allow the holographic
scanner to achieve a much larger depth of field than would otherwise be possible
because of the resultant reduction in the total scan velocity range, and the
accompanying reduction in the required electronic bandwidth.

A holographic bar code scanner can provide a large depth of field by using
the multiple focal plane concept, wherein each holographic facet on the scanning
disk is focused at a different distance from the disk. One problem associated
with such a scanner is the large range of linear scan velocities that are
encountered when going from the shortest focal length scan line to the longest
focal length scan line. The reason for this is that the scanning disk rotates at a
constant angular velocity, resulting in a linear beam-scanning velocity that is
proportional to the distance from the disk.

This increase in scan velocity with distance from the disk places a practical
limit on the depth of field which is less than that imposed by optical
considerations. The large range in scan velocities establishes a large bandwidth
requirement on the analog electronics. Eventually, the electronic bandwidth
requirement exceeds the practical limits of the state of the art circuitry, or the
required bandwidth reduces the electronic gain to an unacceptable level.

It would be desirable to have a scanner in which the angular scan velocity
decreased with distance from the scan disk so that the linear scan velocity would
be nearly constant throughout the entire depth of field.

The equation below relates the scan angle of the deflected beam to the
rota...